The present invention relates to a method of securing a tip in a tappet, and particularly, to a method of securing a wear-resistant metal tip between an engine valve shaft and the top wall of the tappet of a direct-acting valve operating mechanism in an internal combustion engine.
Recently, in order to increase allowable maximum speed and maximum brake power in an internal combustion engine, a DOHC-type valve operating mechanism has been used. In order to make the valve operating mechanism lightened, a tappet (valve lifter) in the valve operating mechanism has been made of an aluminium alloy instead of conventional steel.
Such aluminium alloy tappets have strength, rigidity and wear resistance lower than steel tappets, so that wear resistant metal is applied between a rotary cam and an axial end of an engine valve.
FIG. 10 illustrates a known tappet in a direct-acting valve operating mechanism. The numeral 101 denotes a cylinder head which a tappet body 102 slidably contacts, the tappet being a cylinder the upper surface of which is closed. A circular wear resistant plate 105 made of wear resistant metal fits in a larger diameter circular recess 104 on the upper surface of a top wall 103, and a smaller diameter tip 107 made of wear resistant metal fits in an engage bore 106 on the lower surface of the top wall 103. The lower surface of the tip 107 contacts the axial end of an engine valve connected to a cylinder head 101 by a pair of opposing cotters 108 and 108; a spring retainer 109 which hold the cotters; and a valve spring 110 for urging the spring retainer 109 upwardly. A rotary cam 112 disposed substantially above the axis of the engine valve slidably contacts the upper surface of the wear resistant plate 105.
To fix the tip 107 in the engage bore 106 of the tappet 102, there is a known method as shown in FIG. 11, which illustrates the tappet 12 inverted to one in FIG. 10 to illustrate a position during working. The bottom of the tappet 102 is upwardly placed. The circular tip 107 made of wear resistant metal fits in the engage bore 106, around which a circumferential projection 113 is pressed by a caulking punch 120. On the lower surface of the punch, there are provided a plurality of inverted triangle-sectioned blades 121 on a circle having a diameter larger than that of the engage bore 106. The punch 120 is pressed downwardly, so that the caulking blades 121 push into the upper surface of the top wall 103 of the tappet 102 in FIG. 11. Thus, the circumferential projection 113 around the engage bore 106 is plastically deformed inwardly so as to fix the tip 107.
In the known art, there is a smaller engage area between the circumferential projection 113 around the engage bore 106 and the upper surface(in FIG. 11) of the tip 107, and the circumferential projection 113 has a relatively smaller inward deformation, so that the tip 107 is not sufficiently fixed. If the tip 107 is not sufficiently fixed within the engage bore 106, there is a clearance between the tip 107 and the tappet 102, or the tip 107 and the shaft 111 of the engine valve, which results in clattering during operation, thereby causing one-sided wear. Further if such abnormal movement occurs in the valve operating mechanism, the tip 107 may drop out. To fix the tip 107 firmly, the caulking blade 121 of the punch 120 may be made as large size, thereby providing a large plastic deformation of the engage bore 106. However, in such a case, a crack 114 is liable to be formed at the bottom of a triangle-sectioned groove (hereinafter refer to "caulking groove") on the upper surface of the top wall of the tappet 102. The caulking blade 121 in the known punch 120, for example, comprises a plurality of separate arcs in a circumference as shown in FIG. 12, a bottom view of the punch. In FIG. 12, there are provided six caulking blades 121 the arc of which is subtended over 40 degrees of a central angle at intervals of 20 degrees. The sides of the caulking blade 121 intersect a circumferential portion substantially at a right angle. Accordingly, the sides of the caulking groove in the tappet 102 is formed as a sharp angle as well, so that the crack 114 is liable to be formed.
In order to solve the foregoing disadvantages in the prior are, the object of the present invention is to provide a method of securing a tip in a tappet in an internal combustion engine having high durablity and reliability by increasing caulking force of the tip and fixing the tip in an engage bore of the tappet firmly.
According to one aspect of the present invention, there is provided a method of securing a tip into an engage bore in a top wall of cylindrical tappet body the upper end of which is closed, in an internal combustion engine. comprising the steps of: forming a chamfered portion on the tip at an opening side of the engage bore; and pressing the top wall by a punch having a caulking blade to form a caulking groove around the engage bore and to deform the circumferential projection plastically around the engage bore over the circumferential projection of the tip inwardly.
According to another aspect of the present invention, there is provided a method of securing a tip to a tappet in an internal combustion engine, comprising the steps of: fixing a tip in an engage bore of a top wall in a cylindrical tappet body an upper end of which is closed; and pressing a circumferential projection around the engage bore by a punch having a triangle-sectioned caulking blade which has a concave surface at an inner side to attain plastical deformation of the projection.
According to further aspect of the present invention, there is provided a method of securing a tip to a tappet in an internal combustion engine, comprising the steps of: fitting the tip having a chamfered portion in an engage bore of a top wall of a cylindrical tappet body the upper end of which is closed; and pressing a circumferential projection of the top wall by a punch having a triangle-sectioned caulking blade until an end of the caulking blade reaches to a depth beyond a lower edge of the chamfered portion, thereby making plastical deformation of the projection.
There are advantages of the present invention as follows. The circumferential projection of the top wall of the tappet body is plastically deformed inwardly, and the engaged area between tile tip and the engage bore is increased. Therefore, caulking force of the tip is increased and the tip is firmly fixed in the engage bore. If the chamfered portion is formed on the tip, the contact area between the circumferential projection around the engage bore and the tip becomes larger and adhesiveness is increased, so that the tip is more firmly fixed.